A semiconductor laser (i.e., laser diode, LD) generally has a relatively short lifespan in comparison to other semiconductor components, and an emission controller of the LD, or the like is generally configured to detect the degradation of LD with time, so that the degraded LD is replaced with a new one before it actually breaks down.
In an alternative technique for detecting the LD degradation, the LD is configured such that current flowing in the LD is constantly monitored over time to detect the LD degradation. For example, if the current flowing in the LD is monitored in each process of the semiconductor laser driving device assembly line, which one of the processes where the LD degradation has occurred may be instantly clarified. Accordingly, the LD degradation may be detected immediately and the degraded LD may be replaced with a new one at an earlier stage.
As one example of a related art technology for detecting LD degradation, Japanese Laid-Open Patent Application Publication No. 2000-280522 (also referred to as “Patent Document 1”) discloses an image forming apparatus capable of detecting LD degradation by monitoring current-voltage converted voltages in which a laser diode LD and a resistor R are connected in series so as to monitor the current-voltage converted voltage. Likewise, as another example, Japanese Laid-Open Patent Application Publication No. 09-289493 (also referred to as “Patent Document 2”) discloses an optical transmitter capable of detecting LD degradation by utilizing current flowing in elements of the optical transmitter. The disclosed optical transmitter includes a current mirror circuit configured to generate an LD current (current for causing the LD to emit a laser beam), and a resistor or transistor provided at an input side of the current mirror circuit for limiting surge current by utilizing the current flowing between the provided resistor or transistor and the elements of the current mirror circuit. Further, as still another example, Japanese Laid-Open Patent Application Publication No. 08-295048 (also referred to as “Patent Document 3”) discloses an image forming apparatus capable of detecting LD degradation by providing an LD degradation detector. In the disclosed image forming apparatus, the LD degradation detector is provided with an LD connection terminal to detect the LD degradation.
However, as illustrated in the image forming apparatus disclosed in Patent Document 1, if the resistor R is serially connected to the LD, a resistance load and a parasitic capacitance component are generated in an output current line toward the LD, which may inhibit high-speed operation of the LD driving device. Similarly, in the optical transmitter disclosed in Patent Document 2, a resistance load and a parasitic capacitance component are generated in the input side of the current mirror circuit. Further, in the image forming apparatus disclosed in Patent Document 3, a resistance load may be generated in the LD output line.
Further, in disclosed Patent Documents 1 to 3, since a load is incorporated in a main circuit of the LD driving device, a value of the load may not easily be controlled. For example, if the load is incorporated as a semiconductor circuit in the main circuit, the accuracy in the current monitoring or surge current detection may be lowered due to variability in the resistors and elements. As a result, the emission controller or the like may need to perform electrically complicated fine adjustments in order to improve the detection accuracy, which may result in time-consuming work.